1. Field of the Invention
The present invention relates to a admission control method in wireless networks and, more particularly, to a double-threshold admission control method in cluster-based micro/picocellular wireless networks.
2. Description of Related Art
Wireless networks provide wireless access to mobile users. With the growth of the number of mobile users and the emergence of broadband services, the demand for higher network capacity is increasing. In order to provide higher capacity under limited radio spectrum, micro/picocellular architectures for wireless networks have been proposed. These architectures have the advantages of higher throughput, lower power requirement for transmitters and higher radio frequency reuse due to reduced cell size.
With smaller cell size, the frequency of hand-off events will increase. This will in turn increase the overheads for processing hand-off events. To reduce the overheads due to frequent hand-off, Posner and Guerin proposed to reserve a number of guard channels at each base station for hand-off calls (See E. C. Posner and R. Guerin, “Traffic Policies in cellular Radio that Minimize Blocking of Handoff Calls, ”Proc. 11th Teletraffc Cong. (ITC 11), Kyoto, Japan, September 1985). New calls are limited to use the rest of the channels. Therefore, new calls are not admitted when there are no free channels for new calls. However, new calls can still be admitted under heavy load when active calls hand-off to neighboring cells and release channels for new calls. Eventually, some of the cells may become congested resulting in increased hand-off dropping probability.
In addition, Naghshineh and Acampora proposed a call admission policy for cluster-based micro/picocellular wireless networks such that the call hand-off dropping probability and forced call termination probability can be kept below a predetermined level (See M. Naghshineh and A. S. Acamporo, “Design and Control of Microcellular Networks with QOS Provisioning for Realtime Traffic,” J. High-Speed Networks, vol. 4, no. 5, pp. 53-71, 1996). An admission threshold at the cluster level is used to limit the admission of new calls. When a new call arrives, the call is admitted if the number of active calls in the cluster is less than the admission threshold; otherwise, the call is rejected. The admission threshold can be properly selected such that a predetermined level of call hand-off dropping probability and forced call termination probability will not be exceeded. This policy remedies the drawback of the policy proposed by Posner and Guerin. However, this policy poses another problem. Since new calls arrive at the cells randomly, some of the cells in a cluster may become congested before the number of active calls in a cluster: exceeds the admission threshold. Therefore, the above conventional call admission methods are not satisfactory, and a novel method that can mitigate and/or obviate the aforementioned problems is desired.